A typical conventional process which is generally used for producing a printing plate by the electrophotographic method comprises uniformly charging a photoconductive layer of the electrophotographic printing plate precursor, imagewise exposing the photoconductive layer, wet-developing the precursor with a liquid toner to obtain a toner image, fixing the toner image, treating the precursor with a oil-desensitizing solution (an etching solution) to hydrophilize a non-image portion having no toner image.
Hitherto, paper, etc. which has been made electrically conductive has been used as a support for the above-described electrophotographic printing plate precursor, but water-permeability of the support affects the printing ability and photographic performance. More specifically, the above-described etching solution and a fountain solution used during the printing are permeated into the support thereby causing elongation of the support. In some instances, the photoconductive layer is peeled-off from the support thereby decreasing the press life, and the water content in the support is changed depending upon the humidity and temperature conditions in atmosphere during the above-described charging and exposure, and, as a result, the electrical conductance of the support varies and adversely affects the photographic performance of the precursor. Also, lack of water-resistance generates wrinkle during the printing.
In order to solve the above problems, it has been proposed that one or both side surfaces of the support are coated with a water-resistant resin, for example, an epoxy resin or a copolymer of ethylene and acrylic acid as described in, for example, JP-A-50-138904, JP-A-55-105580 and JP-A-59-68753, or a laminate layer such as polyethylene is provided on one or both side surfaces of the support as described in, for example, JP-A-58-57994. (The term "JP-A" as used herein means an unexamined published Japanese patent application.)
A back coating layer provided on the surface (i.e., side) opposite to the surface (i.e., side) having a photoconductive layer of the support (a printing surface or a top surface) is generally called as "a back layer", and various improvements have been made in the composition for forming the back layer for the purpose of maintaining various functions in addition to the above-described function to impart the water-resistance to the printing plate precursor.
On the other hand, the present inventors have developed a wet-development method which is so-called a direct electrical supply system in place of the conventional system in which a master plate is passed into a developing solution flowing between electrodes. In the wet-development by the direct electrical supply system, an electrically conductive element is used in place of the electrode at an opposite side of the printing surface, and the development is conducted while directly supplying electrons from the electrically conductive element to the back surface of the support opposite to the printing surface, as proposed in JP-A-1-26043. By employing this system, a development rate can be increased, a solid image can be formed uniformly, and further adhesion of a toner on the electrode of back side in the developing apparatus can be prevented.
As a printing plate precursor suitable to the direct electrical supply system, the present inventors also proposed that a layer having a surface electric resistance of 1.times.10.sup.10 .OMEGA. or less and having a frictional resistance higher than that of a polyolefin laminate layer is provided as a back layer of the support which has polyolefin laminate layers on both side surfaces, as described in JP-A-2-84665. By providing the above-described layer, the resulting printing plate can be wound and fixed around a drum of printer precisely, and printing deviation can be prevented whereby a satisfactory electrophotographic printing plate can be obtained, and, at the same time, a printing plate precursor which can be developed by the direct electrical supply system can be produced.
Further, the present inventors proposed a printing plate precursor comprising a support having provided on the surface thereof an underlayer having a surface resistivity of from 1.times.10.sup.8 to 1.times.10.sup.14 .OMEGA. and a photoconductive layer and having provided on the opposite surface thereof a back layer having a surface resistivity of 1.times.10.sup.10 .OMEGA. or less, as well as a method for the development thereof, as described in JP-A-2-132464. The resulting printing plate precursor can be developed precisely, satisfactory and rapidly by the wet-development process of either the conventional process or the direct electrical supply system and is capable of forming a uniform image without generation of pin-holes in the solid image portion.
However, in the above-described JP-A-2-84665 and JP-A-2-132464, a mechanical strength and the water-resistance of the back layer are deteriorated due to the adjustment of the surface electric resistance of the back layer, thereby causing stains in prints and deterioration of printing plate durability.
Further, JP-A-5-6018 and JP-A-5-6036 propose a back layer composed of a plurality of layers, in which a hydrophilic polymer is incorporated into the outermost layer and a layer having water-resistance is used as an inner layer, and it is described that frictional stains during the storage in a laminated state can be prevented and the printing performance can be improved by imparting the water-resistance.
However, in these techniques, the mechanical strength of the outermost layer is still insufficient due to the adjustment of the surface electric resistance of the back layer, and pieces of the back layer peeled-off are transferred to the photoconductive layer thereby causing stains of prints.
Accordingly, at present there are no electrophotographic printing plate precursors and the method of development thereof which satisfy all the requirements of mechanical strength, the water-resistance and the adaptability to a development by the direct electrical supply system.
As a result of extensive studies in order to solve the above problems, the present inventors found that, when the back layer is composed of at least two layers, with the inner layer thereof having a lower electric resistance and the outermost layer having an electric resistance higher than that of the inner layer, the resulting printing plate precursor can be sufficiently developed by the direct electrical supply system as long as the inner layer has a low resistance even if a layer having a certain degree of high resistance is present thereon, and is excellent in the mechanical strength and the water-resistance while enjoying an advantage of the direct electrical supply system.
More specifically, according to the present invention, it was found that an electrophotographic printing plate precursor which is excellent in the water-resistance and the mechanical strength, which is capable of forming a uniform image and which is suitable to the development by the direct electrical supply system can be obtained.